Beagle 2 was declared lost after no communications were received following the scheduled landing on Mars. Attempts at contact were made for over a month after the expected landing at 2:54 UT December 25. A board of inquiry was appointed to look into the reason for the failure and released its report on 24 August 2004. No concrete reason for the probe's failure was determined. Factors that were considered as plausible causes of the failure were an unusually thin atmosphere over the landing site, electronic glitches, a gas bag puncture, damage to a heat shield, a broken communications antenna, and a collision with an unforeseen object.

The Beagle 2 was mounted on the top deck of the Mars Express Orbiter. It was released from the orbiter on 19 December 2003 on a course to land on Mars at 2:54 UT on 25 December. A point at 10.6 N, 270 W in Isidis Planitia, a large flat region that overlies the boundary between the ancient highlands and the northern plains of Mars, was chosen as the landing site.

The lander was expected to operate for about 180 days and an extended mission of up to one Martian year (687 Earth days) was considered possible. The Beagle 2 lander objectives were to characterize the landing site geology, mineralogy, geochemistry and oxidation state, the physical properties of the atmosphere and surface layers, collect data on Martian meteorology and climatology, and search for signatures of life.

Beagle 2 was equipped with a robot sampling arm and a small mole, (Planetary Undersurface Tool, or PLUTO) which could be deployed by the arm and was capable of moving across the surface at a rate of about 1 cm every 5 seconds using a compressed spring mechanism. This mechanism also enabled the mole to burrow into the ground and collect a subsurface sample in a cavity in its tip. The mole was attached to the lander by a power cable, which could be used to pull the sample back to the lander. The lander was equipped with instruments for gas chromatography and mass spectroscopy (the Gas Analysis Package, or GAP), a microscope, panoramic and wide-angle cameras, Mossbauer and X-ray flourescence spectrometers and environmental sensors.

The robot arm was equipped with a grinder and corer, a device to collect a core sample from inside any rock within reach of the robot arm. The Mossbauer and X-ray spectrometers and the microscope were also held in a package on the end of the arm called the position adjustable workbench, or PAW. A stereo camera system was also mounted on the arm. The lander was shaped like a shallow bowl with a diameter of 0.65 m and a depth of 0.25 m. The cover of the lander was hinged and folded open to reveal the interior of the craft which held a UHF antenna, the 0.75 m long robot arm, and the scientific equipment. The main body also contained the battery, telecommunications, electronics, central processor, and the heaters. The lander was to gather power with four disk-shaped solar arrays. The lander package had a mass of 69 kg at launch, but the actual lander is only 33.2 kg at touchdown.

Beagle 2 was launched with the Mars Express orbiter and was released on a ballistic trajectory towards Mars on 19 December 2003 at 8:31 UT. Beagle 2 coasted for five days after release and entered the Martian atmosphere at over 20,000 km/hr on the morning of 25 December. As no signals were received after separation from Mars Express it is not known what happened during the landing sequence.

After initial deceleration in the Martian atmosphere from simple friction, parachutes were to be deployed and about 1 km above the surface large gas bags would have inflated around the lander to protect it when it hit the surface. Landing was expected to occur at about 02:54 UT on 25 December (9:54 p.m. EST 24 December). After landing the bags would deflate and the top of the lander would open. The top would unfold to expose the four solar array disks. Within the body of the lander a UHF antenna would have been deployed. A panoramic image of the landing area would be taken using the stereo camera and a pop-up mirror. A signal was scheduled to be sent after landing (and possibly an image) to Mars Odyssey at about 5:30 UT and another the next (local) morning to confirm that Beagle 2 survived the landing and the first night on Mars. No signal was received at this time nor at any of the subsequent opportunities.